It is well known in the medical arts to provide a metallic bone prosthesis with a porous metallic coating to enhance the fixation of the prosthesis to the patient's bone. Such fixation is generally achieved by either cementation or tissue ingrowth, or a combination of these techniques. The bone cement or freshly grown bone tissue occupies pore volume in the porous coating and thereby serves to lock the prosthesis in place. Fixation by tissue ingrowth has been recommended by many workers in the field of orthopedics as a means of eliminating or alleviating several disadvantages associated with fixation by cementation (e.g., premature loosening of the prosthesis, tissue reaction with the bone cement, the need to remove a substantial amount of the patient's bone to provide space for layer of bone cement). Failure of a bone tissue ingrowth fixation, which would of course lead to a premature loosening of the prosthesis, remains as a matter of concern. In a metallic prosthesis comprising a porous coating extending over a non-porous base portion, such failure can occur at the base portion-porous coating interface, at the porous coating-bone interface, within the porous coating, or within the patient's bone outside of the coating.
A variety of different porous metal coatings have been proposed in the prior art for enhancing fixation of a metallic prosthesis by bone tissue ingrowth. Thus, for example, U.S. Pat. No. 3,855,638 discloses a surgical prosthetic device, which may be used as a bone prosthesis, comprising a composite structure consisting of a solid metallic material substrate and a porous coating of the same solid metallic material adhered to and extending over at least a portion of the surface of the substrate. The porous coating consists of a plurality of small discrete particles of metallic material bonded together at their points of contact with each other to define a plurality of connected interstitial pores in the coating. The size and spacing of the particles, which may be distributed in a plurality of monolayers, is such that the average interstitial pore size is not more than about 200 microns, an essential limitation according to the patentee. Additionally, the patentee teaches that the pore size distribution should be substantially uniform from the substrate-coating interface to the surface of the coating. Failure of the bone tissue ingrowth fixation of the prosthesis disclosed in U.S. Pat. No. 3,855,638 tends to occur at the coating-bone interface or in the patient's bone outside of the coating.